Photodetectors are devices that produce electrical signals in response to optical signals, and they are used in many aspects of modern technology. For example, photodetectors are used in optical astronomy, photovoltaic solar energy conversion devices, and as light detectors in optical communications systems.
A presently constructed and contemplated, optical communications systems will comprise a light source, and as a light emitting diode or a laser, and a photodetector which are optically coupled to each other through a glass transmission line that is commonly referred to as an optical fiber. Optical fibers presently used are silica based and have low losses in the region between 1.0 .mu.m and 1.6 .mu.m, and it is therefore believed that many of the future optical communications systems will operate within this region to take advantage of the low fiber losses in this region, although systems operating at both shorter and longer wavelengths will undoubtedly be constructed.
There are several types of photodetectors that can be used in optical communications systems. For example, photodiodes, phototransistors and avalanche photodiodes have been considered for use as photodetectors in such systems. Regardless of the particular type of photodetector selected, the photodetector must satisfy certain systems requirements such as adequate sensitivity at the wavelength or wavelengths of interest, adequate response time and as little noise as is possible.
Avalanche photodiodes are attractive candidates for use in optical communications systems because they typically have excellent response times and high gains. Their drawbacks generally include operation at relatively high voltages and an increased noise level which results from the avalanche multiplication process. It has long been realized, see, for example, R. J. McIntyre, IEEE Transactions on Electron Devices, ED-13, pp. 164-168, January, 1966, that low noise may be achieved at high gains by having a large ratio of the ionization coefficients (.alpha.,.beta.) for electrons and holes. The optical communications systems presently operating near 0.8 .mu.m generally use photodetectors based on low noise silicon avalanche photodetectors which have a .alpha./.beta. ratio of approximately 50. Photodetectors for use between 1.0 .mu.m and 1.6 .mu.m will undoubtedly be based on Group III-V compound semiconductors and avalanche photodetectors using these compound semiconductors are under development. Unfortunately, the .alpha./.beta. ratio for most Group III-V compounds is approximately unity and the result is increased avalanche multiplication noise. Thus, devices having increased .alpha./.beta. ratios are of considerable practical importance.
Several avalanche photodetector structures have been proposed which enhance the .alpha./.beta. ratio. For example, a graded bandgap avalanche photodetector with .alpha. equal to approximately 5.beta. to 10.beta. has been fabricated by grading the avalanche region so that the probability of electron ionization is greatly increased while that for hole ionization is not. This photodetector was described at the Symposium on GaAs and Related Compounds held at Oyso, Japan in September 1981. A superlattice avalanche photodetector is proposed in Electronics Letters, 16, pp. 467-469, June 5, 1980. This article describes a device with an enhanced .alpha./.beta. ratio. The ratio may be as high as 20 and is obtained by band edge discontinuity assisted impact ionization using alternating wide and narrow gap layers. An enhanced ionization rate for electrons was obtained by having a discontinuity in the conduction band that is greater than the discontinuity in the valence band. Quantum wells were to be formed by narrow gap layers between wide gap layers. Increasing the number of quantum wells, at least up to 30 or 40 quantum wells, was expected to result in enhanced .alpha./.beta. ratios. A still larger number of quantum wells should produce essentially no increase in the ratio. However, the valence band discontinuity, at the trailing edge of the well, would be such that holes would inevitably ionize and therefore increase the noise in the avalanche process. Further, the conduction band discontinuity was necessarily small to avoid electron trapping in the quantum wells. This limits the achievable electron ionization enhancement and also requires the use of fields high enough to supply the remainder of the ionization energy. This also results in appreciable hole ionization.